gtkdoc-scan: Fix use of uninitialised value in trace logging

Fixes:
> Use of uninitialized value $6 in concatenation (.) or string at
> /opt/gnome3/build/bin/gtkdoc-scan line 552, <INPUT> line X.

This was caused by re-using the numbered match variables from one regexp
after running another regexp.

https://bugzilla.gnome.org/show_bug.cgi?id=724002